This invention relates to a telemetry system for measuring strain on structural members of rotating machinery during operation, and more particularly to a subcarrier telemetry system that is self-calibrating and in which low-frequency dynamic strain signals are separated from background noise associated with rotational speed of the machine.
Strain resulting from operational stress on a machine is ordinarily measurable by strain gaging techniques well known in the art. However, with machines having rotating members such as, for example, a turbine rotor, transmission of the measured information presents a special problem in establishing a reliable transfer link between the rotating objects of the measurement and the stationary data collection point. Slip rings to establish electrical contact between rotating and stationary members, and through which information signals might flow, are of little use in the operating environment of a high-speed turbine. The measured information, in the form of an electrical signal, is virtually inseparable from other, spurious noise signals introduced as the contact brush bears upon its slip ring. In many instances, too, the use of slip rings would necessitate an unacceptable modification to the turbine itself.
Radio-telemetry is presently the most practical means available for gathering information on the strain produced in turbine rotors during operation, and it has been used for that purpose since the late 1950's. The obvious advantage of telemetry is that a wireless link is provided between rotating sensing devices and stationary instrumentation. A discussion of radio-telemetry for the measurement of strain on turbine rotors is available in Sound and Vibration, April 1973, by V. Donato and S. P. Davis under the title "Radio Telemetry for Strain Measurements in Turbines".
In spite of their history of utility, however, telemetry systems continue to suffer certain severe limitations. As applied in turbine measurements, there is an unavoidable susceptibility to magnetically induced noise signals which are synchronous with the speed of turbine operation. This noise phenomenon is a result of the strain gages, lead wires, and transmitters being rotated at high velocities through regions where small residual magnetic fields cannot be eliminated. Repeated identically each revolution, the noise signals are indistinguishable from vibrational strains produced by a "per rev" stimulus.
Another major limitation of telemetry systems is the poor accuracy of the measurement information gathered. This results to a large extent from variations in overall system sensitivity. For example, variations in the gain of the telemetry transmitter during operation is one significant factor limiting the accuracy attainable. The apparent magnitude of the strain will be observed to go up or down depending on the instantaneous gain of the transmitter. Temperature plays a part in this, being known to affect both transmitter gain and other system components so that an uncertain portion of the apparent gage response may be due to temperature and not truly due to strain.
Accordingly, it is an object of the present invention to provide an improved radio-telemetry system in which the desired, measured information is separated from undesired noise signals to obtain more reliable information.
An additional object of the invention is to achieve more accurate measurements by radio-telemetry, providing means for self-calibration to eliminate the effects of extraneous factors which may affect overall system sensitivity and gain.